Impacts of source regions and atmospheric transport on physical properties of black carbon and tracer ratios over the Yellow Sea: evidence from multi-seasonal airborne observations

Abstract. Both size and mixing state of Black Carbon (BC) are critical climate-relevant physical parameters. It remains a challenge for ambient measurements to characterize their variability across different atmospheric conditions particularly in outflow regions. To investigate how BC's physical properties are determined in source regions and altered during transport, we conducted 23 flight measurements of BC, CO, and CO2 over the Yellow Sea from 2021 to 2022. The refractory BC mass concentration (MrBC) varied by up to two orders of magnitude between near sea surface and around 5 km above sea level, and Planetary Boundary Layer height-dependence of MrBC was stronger in winter than in spring. Smallest rBC's mass median diameter (MMD, 163.4 nm) observed in South Korea-sourced air indicated fresh urban emissions, whereas larger MMD, enhanced internal mixing, and higher rBC/CO slopes were exhibited in the air masses from North Korea and China, reflecting additional emissions from biomass and coal combustion. Both MMD and internal mixing tended to decrease with altitude, highlighting the wet scavenging effect during particle transport. When accumulated precipitation exceeded 1 mm, MrBC decreased by more than 50 %, with moderate decrease in MMD and internal mixing. As a result, overall BC transport efficiency declined to 1/e within 5.5 d. These observations reflect the complex effects of source region, seasonality, and wet removal on varying rBC distributions in the outflow region. These findings provide valuable observational constraints for improving model representations of the size distribution and mixing state of ambient BC particles in outflow regions.